Ring oscillators are used in a variety of circuit applications. For example, ring oscillators may be used in clock circuits and delay-locked-loop-type circuits. A conventional ring oscillator is formed from a loop of inverters. There may be, for example, hundreds of inverters in the ring oscillator, each of which has its output connected to the input of another one of the inverters. In some designs, a NAND gate may be inserted into the loop in place of one of the inverters. One of the NAND gate inputs may be used as an enable input. The ring oscillator may be enabled by asserting a trigger signal on the enable input. When the value of the trigger signal on the enable input is low, the ring oscillator will be turned off and will not oscillate. When the value of the trigger signal on the enable input is high, the ring oscillator will be enabled and will oscillate.
The trigger signals that are used for enabling and disabling ring oscillators in this way are generally produced using off-chip test equipment. As the trigger signal is routed to the enable input of the NAND gate through interconnects, the trigger signal can become degraded. In particular, a square wave trigger signal may pick up undesirable ringing characteristics due to parasitic circuit elements or due to power supply glitches. The spikes or other noise characteristics that are present in a trigger signal that has been degraded in this way may cause a ring oscillator to enter undesirable modes of operation in which higher-order harmonics propagate around the loop. When this occurs, the operation of the ring oscillator may be unstable or the output of the ring oscillator may oscillate at an undesired higher-order harmonic frequency rather than at an intended fundamental frequency.